Stereo image masking

Critiquing a few mixes this week before final submission for mastering I encounter a common issue with the use of stereo in the mixing process.

Digital Audio Work Stations (DAW’s) as a mixing platform positively providing an excellent set of tools and flexibly during production. They also negatively lead to a lack of focus in some areas of mix construction – Correct use of stereo image being one example.

As many DAW’s create channel paths by default there can be a tenancy to ignore the spatial routing. If it’s a mono sound source it routes to centre of left / right (mono). If it's a ‘stereo’ instrument or effect it routes to left / right (stereo). This is normally actually mono left and mono right as there is little or nothing panned between these two points, as often these sounds are a pseudo stereo source.

This is where the source is in fact mono but creates a ‘stereo’ image by the use of phase, in terms of delay between left and right and / or equalisation difference. One side is brighter or darker than the other. When this folds to mono it often cancels to some extent, or causes a phased aspect to the sound. Some virtual instruments even use polarity to give an impression of width in the effect elements in their output which obviously cancels in mono! Don't be fooled by a plugin or auxiliary routing path, just because it say's it has a stereo input doesn't mean it will actually process the effect elements in stereo.

The dry path will route through the plugin in stereo but the wet element will have summed to mono, then processed the effect in mono creating a pseudo stereo output to mix in with the original dry elements. Testing is the only way to know for sure what the processing nature of a plugin path is.

A good example of a sound creating masking by occupying all the stereo image is a virtual instrument piano. This is normally placed as wide across the stereo as possible, bass through to treble in register, left through to right. It can sound great when played in isolation but occupies the whole stereo field when in a mix. Manipulation of the left right pan controls to balance in to the stereo field will often achieve a more cohesive sound and better mono compatibility. Creating a mini stereo inside the big stereo, what I like to refer to as ‘pocketing’. Equally this can allow for the ambience applied to sit outside the sounds stereo image giving a sense of dimension to the overall piano - more 'real'.

The other part of the problem is if these ‘stereo’ effects and instruments set hard left right and all the other mono aspects are in the centre. We don’t actual have ‘real’ stereo, the mix is made up of mono left, mono right and mono centre. Basically we missed out everything in between. Masking is taking place across all elements in the centre and left / right aspects, as they are stacked up on top each other. Much as in the same way if two sounds are playing differing melodies in the same octave they’d cover each other in some aspects, move away by an octave and they become distinct – unmasked.

A mix in this masked state does not sound bad, it’s just not accessing all of the possible detail that could be achieved with the simple application of good panning practice, hence unmasking these elements.

If you find this description familiar, even a basic approach towards placing your stereo elements at differing stages of pan across the image using a dual panner mode on your DAW will have a big impact. There’s always a way to achieve control of pan for each mono element of the stereo parts, even if this means printing the audio in split stereo and loading back to mono tracks.

Once control is achieved you should think of each stereo element as a ‘mini’ stereo that sits in the ‘big’ stereo mix. This can be is as simple as placing your effects L100 / R100, L95 / R95, L90 / R90 and so on, this will create a wider stereo image and clearer detail overall. If you bounce a version of each mix and AB, you’d clearly hear the detail difference as your mix opens out. Even though there is less in the hard stereo the mix will appear wider. This is just simple unmasking.

Don’t forget the mono aspects!

Moving parts placed dead centre even a few points can achieve more clarity. A good rule of thumb is no element should be at the same pan position as another. Though only a guide it’s a good way to get your head in to the effective use of pan. There are obviously some elements that might want to stay dead centre, kick, snare, bass, vocal. Though I still find, as have others, manipulation of these points can achieve a denser mix.

In terms of mastering this means not only is the mix clearer, hence needing less processing, but the overall depth / density is fuller, which will help with apparent loudness.

As I said many times before; loudness is achieve through effective mixing not the use of compression and limiting during the mastering process.

Another good approach to panning a mono aspect when there is a lot going on in the mix is to move the sound around the whole image to find the point where the sound is clearest - i.e. least masking is taking place. You can also apply this technique with the monitoring set to mono. Simply referred to as 'panning in mono' moving the pan around again to hear the point where the sound is clearest, again least masking and phase cancellation is taking place.

Taking this idea of separation to the next level you can use buses to control sets of instrumentation and their effects in the overall stereo field. Very effective for strings, brass, guitars, drums, piano etc. You can not only control the apparent width but also centre focus of that image in the overall ‘big’ stereo. This concept which I like to call ‘pocketing the buss’ I’ll discuss in a future article.

One note; don’t be tempted to use a ‘stereo width or imaging’ tool.

This often can introduce unwanted phase in to the mix and, on the whole, does not allow you to actually change the left right panning position, just the width. One exception to this is ‘Flux Stereo Tool’ which has excellent manipulation of pan position.

To summaries; there’s never a correct answer but experimentation and application of method will deliver rewards in term of the overall mix clarity every time.